1. Field of the Invention
The present invention relates to automatic exposure control, especially to an automatic exposure control device in a single lens reflex camera for continuous photographing.
2. Description of the Prior Art
When exposure control is effected by full-open photometry or an actual aperture photometry in a single lens reflex camera with an automatic exposure control device for continuous photographing, a diaphragm aperture is controlled from the full-open aperture of the lens to a regulated aperture and an exposure is made by a shutter opening action. After an elapse of any length of control time to make an appropriate exposure, the shutter makes a closing action, then the diaphragm aperture is returned to a full-open aperture position again. That is, after a completion of one exposure action, a diaphragm aperture is always returned to a full-open aperture position and is to be stopped down again at the time of a subsequent exposure action in a continuous photography. For example, in the case of an exposure control by a full-open photometry, an example of a conventional continuous photographing action is performed in the following manner:
(1) photometry of a light beam passing through a lens is initiated during the state of a full-open aperture by a first switch, and an exposure control value is displayed within a view finder, then a release signal is generated as a second switch is turned ON.
(2) full-open photometry for an exposure control is performed by the release signal thus generated, for memorizing the photometry value.
(3) a diaphragm aperture is stopped down at the same time with a mirror up to a controlled aperture value based on the memorized photometric value or a predetermined set aperture value by a release of a first fastening.
(4) and an opening and closing action of a shutter is performed with a control time based on a predetermined set time or the above-mentioned memorized photometric value.
(5) after that the diaphragm is returned to the full-open position.
(6) and, a film is mechanically loaded by a film wind up action for a subsequent photographing, and at the same time one frame of the film which has been exposed is advanced.
(7) during this state, the state mentioned above for the paragraph (2) is resumed, then the above-mentioned series of actions are repeated, thus making a continuous photographing.
As has been described above, a method for returning the diaphragm to the full-open position every time the exposure is made has been a great obstacle in enhancing the speed of continuous photographing when a diaphragm mechanism with poor responsive characteristics is used or when the responsive characteristics of the diaphragm mechanism become poor in a low temperature circumstance. Especially in a diaphragm mechanism using a step motor, since a length of time required for control in every step becomes almost constant, as the number of steps for aperture control becomes larger, that is, as the number of aperture steps is larger, a longer control time is needed. Such shortcoming becomes conspicuous in a physical diaphragm using a liquid crystal diaphragm or EC (electro-chromi diaphragm), because of the limited response time. And when adverse effect in the low temperature circumstance is also imposed on such a diaphragm, the shortcoming becomes still greater.